1. Field of the Invention
The present invention relates to an image forming apparatus of an electrophotographic type and such as a copying machine, a printer and the like electrostatic recording type.
2. Related Background Art
For an electrifying (or electrostatic charge) device for charging a photosensitive member, there are available a corona electrifying device, a fur brush (see B in FIG. 11), an electrifying roller (see A in FIG. 11) and the like. They are of an electrifying method primarily utilizing electric discharging phenomena.
Instead of such an electric discharging method, an injection charge method for charging without accompanying a discharge is under consideration by directly injecting a charge. With respect to the injection charge method, there is such a method available where an electrical conductive electro-magnetic brush is rubbed against the photosensitive member.
FIG. 11 is a graph showing an example of an electrification efficiency. A bias applied to a contact electrifying member is shown on a transversal axis and a photosensitive member electrification potential is shown on the axis of ordinates.
In the injection charge method, instead of using the magnetic brush method, implementing of the electrification is under consideration by interposing electrical conductive particles (hereinafter referred to as electrification accelerating particles) in the contact portion to accelerate the electrostatic charging by improving the contacting ability by allowing an image bearing member and the contact electrifying member to have a peripheral speed difference.
By this method, it is possible to obtain an electrostatic property equal to or more than that of the electromagnetic brush C as shown in FIG. 11.
When such an electrifying method is used for a cleaner-less apparatus, which eliminates the needs for a special cleaner by recovering transfer residual toners by a developing device, an abutting pressure between the contact electrifying member and the image bearing member is increased by interposing the electrification accelerating particles in a contact nip portion between an image bearing member and a contact electrifying member with a contact torque being reduced so that no transfer residual developers pass through the contact nip portion between the contact electrifying member and the image bearing member.
However, if there exist developers which are not transferred on a transferring material, not only the image bearing member but also transfer residual developers should be given a proper charge. When transfer residual developers have an improper charge, transfer residual developers in a developing device can not be recovered and no excellent image quality can be obtained.
Moreover, when the electrification accelerating particles are excessively supplied to the image bearing member, there arises an adverse situation where they are transferred on a print image or they interrupt an image exposure.
An object of the present invention is to provide an image forming apparatus capable of maintaining an injection chargeability over a long period.
Another object of the present invention is to provide the image forming apparatus with a high recovery efficiency of transfer residual toners by a developing device.
Still another object of the present invention is to provide the image forming apparatus capable of unifying the electrifying polarities of the transfer residual toners.
Still another object of the present invention is to provide an image forming apparatus comprising:
an image bearing member for bearing an electrostatic image;
developing means for developing the electrostatic image on the image bearing member by toners charged with a predetermined polarity;
transfer means for transferring a toner image on the image bearing member to a transferring material;
charging means for inject-charging the image bearing member having transfer residual toners ;
an electrical conductive member provided and spaced apart from the above described image bearing member downstream of the above described transfer means and upstream of the above described charging means in the moving direction of the above described image bearing member; and
electric field forming means for forming an alternating electric field between the above described electrical conductive member and the above described image bearing member.